1. Field of the Invention
This invention relates generally to a belt or chain tensioning device and particularly concerns a tensioning device which accommodates thermal expansion between a series of pulleys engaged with a common belt or chain.
2. Description of Prior Developments
Belt tensioning devices are well known and exist in numerous forms. A common arrangement includes a tensioning member, commonly a torsion spring, which urges a belt tensioning idler pulley against an endless belt to control the tension in the belt and to maintain the engagement of the belt with its associated pulleys. It is necessary to control belt tension in order to prevent the belt from slipping about the pulleys over which the belt travels and for controlling belt wear.
Some tensioning devices have been designed with a fixed idler pulley while others have been designed with a movable idler pulley provided with a motion limiting ratchet and pawl. The ratchet and pawl mechanism prevents an endless belt such as a timing belt or accessory drive belt from overcoming the biasing force of the tensioning member. When the biasing force is overcome, he belt can lose synchronization or slip over the pulleys thereby causing a loss of timing or traction.
An example of a tensioning mechanism incorporating a ratchet and pawl is disclosed in U.S. Pat. No. 4,634,407. While this device performs satisfactorily, an additional need has been found for accommodating the thermal expansion of the base or support upon which the pulleys and the tensioner mechanism are mounted. For example, when a series of pulleys are mounted to an engine block, the relative distances between the pulleys increases as the engine heats up and thermally expands. The result of this thermal expansion is a significant increase in belt tension as the belt is stretched over the pulleys.
Without some form of tension relief, the belt can be over stressed to a point where it is damaged and its useful life is shortened. This problem is particularly acute in those belt drive systems which incorporate a fixed idler pulley or ratcheting mechanism in their tensioners. The fixed position of the idler is typically maintained either by a static pulley mount or by a ratchet, both of which prevent the tensioner from relieving the increased belt tension caused by thermal expansion. In those belt tensioning systems without a ratcheting mechanism, the stretched belt simply displaces the idler pulley by overcoming the force of its tensioning element thereby relieving the belt tension to some extent. This relief has not been available with tensioners which incorporate ratcheting mechanisms.
Accordingly, a need exists for a belt tensioner of the type which includes a ratcheting mechanism for taking up belt slack caused by belt stretching and seating yet which provides for a degree of belt tension relief needed during thermal expansion between the pulleys over which the belt travels.